DENVER—A United Nations-based drug agency urged the U.S. government on Tuesday to challenge the legalization of recreational marijuana use in Colorado and Washington, saying the state laws violate international drug treaties.

The International Narcotics Control Board made its appeal in an annual drug report. It called on the U.S. government to act to "ensure full compliance with the international drug control treaties on its entire territory."

"The entire international system is based on countries respecting the rules, and there's a broad fabric of international treaties that are part and parcel to that," added David Johnson, U.S. delegate to the Vienna-based board.

Last fall, Washington and Colorado became the first states to pass laws legalizing marijuana. Pot remains illegal under federal law.

U.S. Attorney General Eric Holder said last week he was in the last stages of reviewing the Colorado and Washington state laws. He was examining policy options and international implications of the issue.

The federal government could sue the states over legalization or decide not to mount a court challenge.

The International Narcotics Control Board is the independent monitoring body for the implementation of United Nations drug control conventions. Its head, Raymond Yans, called on Holder to challenge the state laws soon after voters approved the measures.

In Amsterdam, where marijuana sold openly in coffee shops has been a tourist draw, officials have cracked down on growers in recent years, earning praise from the control board.

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Brian Vicente, co-author of the Colorado pot legalization law, said a handful of North American countries have expressed support for legalization. He will head to Uruguay next week to talk about pot.

"The United States has been the main driver on this global war on drugs and they've seen significant shifts in their own ranks," Vicente said. "You have two states revolting and they're saying it doesn't work in their state and their community and it sends a strong message globally."

Kasia Malinowska-Sempruch, director of the Open Society Foundations' Global Drug Policy Program, blamed repressive drug laws for millions of arrests and called on the United Nations General Assembly to reconsider its approach when it holds a special session on drugs in 2016.

The U.N. report also cited prescription drug abuse as a continuing problem as well as the emergence of designer drugs engineered to avoid existing drug controls.